Heater system for fiber placement machine

ABSTRACT

Closed-loop systems and methods for controlling the temperature at the compaction point as an automated fiber placement (AFP) machine is placing material over complex surface features at varying speeds. The closed-loop system starts with multiple infrared temperature sensors directed at the layup surface in front of the compaction roller and also at the new layup surface behind the compaction roller. These sensors supply direct temperature readings to a control computer, which also receives speed data and a listing of active tows from the AFP machine and is also programmed with the number of plies in the current layup. In accordance with one embodiment, the heater control system uses a proportional-integral-derivative loop to control the temperature at the compaction point (e.g., at the interface of the compaction roller and a newly laid tow) and regulate the heater power to achieve the desired temperature.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract numberNNL09AA00A-2A38 awarded by NASA. The Government has certain rights inthis invention.

BACKGROUND

This disclosure generally relates to automated fiber placement (AFP)machines that rely on calibrated heaters to control power output. Inparticular, this disclosure relates to systems and methods forcontrolling the heater output during placement of tows offiber-reinforced plastic material.

Fiber-reinforced composite materials comprise fibers embedded in amatrix material, such as thermoset and thermoplastic polymer resins. Thefibers carry loads and provide strength and stiffness. A compositematerial has high strength and stiffness in the direction of the fiber,and lower strength and stiffness in a direction perpendicular to thefiber.

A variety of machines exist that can deposit materials made ofreinforcement fibers pre-impregnated with thermosetting or thermoplasticresin (also known as “prepreg composite material”). Advanced fiberplacement (also known as “tow placement technology”) is a fullyautomated process for the production of composite laminates from aplurality of narrow prepreg tapes, or “tows”, that combines thedifferential payout capability of filament winding and the compactionand cut-restart capabilities of automated tape laying. Carbon fiberspre-impregnated with thermoset resin are most commonly used in theaerospace industry and therefore the fiber placement process will bedescribed herein assuming a thermoset material system.

Most fiber placement systems have seven axes of motion and are computercontrolled. The axes of motion, i.e., three position axes, threerotation axes and an axis to rotate the work mandrel, provide the fiberplacement machine flexibility to position the fiber placement head ontothe part surface, enabling the production of complicated compositeparts. During the fiber placement process, tows of slit prepreg tape areplaced on the surface in bands of parallel fibers, called courses (i.e.,each course consists of multiple parallel tows). The AFP head lays downsuccessive courses to form the multiplicity of layers or plies making upthe final composite laminate.

The major process parameter for controlling the tack and adhesiveproperties of the prepreg system during fiber placement is substratetemperature (that is, the temperature of the prepreg material alreadyplaced on the tool). The substrate will build up to include a pluralityof layers of prepreg material on that tool surface as the laminationprocess proceeds. Automated fiber placement (AFP) machines use heaters,such as infrared heaters in front of the compaction roller to heat thesubstrate in order to enhance material tack prior to laminating a newply over the substrate. Infrared heating provides substantial benefitsin safety and ease of implementation over laser heating sources, andproduces a more robust and effective means of heating compared to hotgas impingement that was first used in the industry. The heat is neededto cause the material to adhere to the surface during the layup ofthermoset composites. An infrared heating system should heat thesubstrate sufficiently to establish good tack without overheating.

One method of heater control uses a calibrated curve of heater power asa function of machine laydown velocity. Typically, during machineinstallation, a heater characterization test is run to measure theresponse of the substrate temperature to various power settings. Aftersweeping through a range of processing conditions, a response table isestablished that defines the commanded heater power output as a functionof machine velocity. The settings are then defined in the machineoperations documentation, such as process control documents, prior touse in production. This is a first-order, open-loop solution that cannottake into account all of the relevant variables that affect the actualmaterial temperature, such as number of plies under the substrate, thetool material, the emissivity of the substrate, heat buildup in thelaminate during continuous processing, heat buildup in the heaterassembly and compaction roller during continuous processing, ambientconditions, and dynamic set points as a function of velocity, amongothers. Additionally, the existing methods of calibrating the heaterstailor the process for the peak temperature of the material which occursbefore the compaction roller. The material then cools a certain amountbefore compaction, resulting in an incorrect understanding of materialtemperature at the compaction point (that is, the location where theincoming new ply contacts the substrate).

The industry typically employs open-loop controls for infrared heatersdue to simplicity and the expediency in qualification of machines forproduction. The lamp used to heat the substrate during AFP currentlyemploys an open-loop control method based entirely on a single heatsetting from the operator and the speed of the head. This limits theability to control heating of complex geometry parts.

SUMMARY

The subject matter disclosed in some detail below is directed to systemsand methods for controlling heater output during automated tow placementbased on a combination of sensor feedback, process models, numericalcontrol (NC) programming data, ambient conditions, and material modelsto determine the optimal power output of the heater. More specifically,this disclosure is directed to closed-loop systems and methods forcontrolling the temperature at the compaction point (a.k.a. “nip point”)to prevent over- and under-heating as an AFP machine is placing tows offiber-reinforced plastic material over complex surface features atvarying speeds. These systems and methods provide active control ofmaterial temperature under the compaction roller during the fiberplacement process, accounting for variability encountered duringprocessing. The primary variable that controls part quality for fiberplacement is the material temperature during compaction. The systemsdisclosed herein control this primary parameter using a closed loop suchthat optimal processing conditions can be maintained during thefabrication of composite structures in order to provide the best qualityat the highest laydown rates possible.

A closed-loop heater control system offers an improvement over a typicalopen-loop method. The temperature of the layup (i.e., substrate) ismeasured in real-time by one or more temperature sensors. The heatercontrol system can achieve the desired temperature in spite of processvariability. The room temperature, initial layup temperature and toolingtemperature can all be sources of process variation for which theclosed-loop system may compensate. Closed-loop control of the heaterallows for control over the manufacturing process as the geometry of AFPparts increases in complexity and capacity. The variables that affectactual substrate temperature include tooling material, number of plieson the surface, substrate temperature, compaction roller material,compaction roller temperature, lay-down rate, infrared heater lag,heater housing temperature, compaction level of the substrate, anddistance of the heater to the surface due to contour variations. Theclosed-loop system can be built around the most fundamental variables.Sensor feedback can be integrated to drive the proper substratetemperature for the ideal lamination process.

More specifically, the embodiments of a closed-loop heater controlsystem disclosed herein incorporate inputs from one or more temperaturesensors (e.g., pyrometers), machine speeds and position, thermal models,and NC program inputs to maintain the optimal processing conditions forthe placement of fiber-reinforced plastic materials, such as thermosetprepreg materials. The system processes the inputs in real-time andoutputs the heater power needed to reach the optimal materialtemperature at the compaction point. The substrate temperature under theheater can also be determined. This information can be used to preventthe material from exceeding maximum allowable temperatures in additionto controlling the temperature at the compaction point.

The closed-loop systems disclosed herein use pyrometers to control thecompaction point temperature in real-time as a new tow or plurality oftows is being placed. In accordance with one embodiment, thermal modelsof the layup, tooling, compaction roller and heater were developed.These are dynamic models incorporating substrate temperature, layupspeed and distance of the heater to the substrate. In addition, thenumber of plies in the current layup will be included to account forvariations in thermal flow. Thermal modeling is used to estimate thetemperature at the compaction point and then adjust heater output.Desired heater output will be a result ofproportional-integral-derivative (PID) control, or other known controlstrategy, and may be driven by geometry, layup speed, and material.

More specifically, the closed-loop heater control system disclosed insome detail below comprises a control computer that uses atwo-dimensional (2-D) thermal model that can correlate temperaturemeasurements from one or more temperature sensors to the actualcompaction point temperature. That compaction point temperature is usedas the control point. The thermal model takes into account variablessuch as substrate temperature, number of plies, tool material, feedrate, distance separating the heater and the substrate, and heaterpower. To construct the thermal model, correlation curves were generatedto relate compaction temperature to the temperature sensor measurements.These correlations are used in real-time in the control loop to controlsubstrate temperature at the compaction point. Significant cooling canoccur from the point where the sensor measures substrate temperature towhere the material is compacted. For advanced AFP processes, knowing theactual compaction point temperature is important to maximize partquality and process capabilities.

In accordance with some embodiments, multiple infrared temperaturesensors (hereinafter “IR temperature sensors”) are directed at the layupsurface in front of the compaction roller and also at the new layupsurface behind (i.e., aft of) the compaction roller. In accordance withalternative embodiments, only one or more IR temperature sensorsdirected at the layup surface in front of the compaction roller are usedor only one or more IR temperature sensors directed at the layup surfacebehind (i.e., aft of) the compaction roller are used.

As used herein, the term “IR temperature sensor” means anoptical-electronic sensor that converts impinging infrared radiationinto a temperature measurement. Such an IR temperature sensor has theability to measure temperature without touching an object. These sensorssupply direct temperature readings to a control computer that controlsthe power supplied to an infrared heater. The IR temperature sensorsshould be mounted and shielded in such a way to prevent photons from theinfrared heater from being reflected directly into the sensors.

In addition, data from robot and NC programs are integrated into thecontrol process. The control computer is communicatively coupled to arobot controller so that it may receive data from the robot. The controlcomputer and robot controller are programmed to pass data and calculatea final heater power output. This output is sent to a heater powercontroller for real-time modulation of heater power. The thermalmodeling uses the layup speed as an input variable. This will besupplied by the robot controller continuously throughout the layup. Inaddition, the part and head geometries influence which, if any, sensorsare pointed at the part. This data will be sent from the robotcontroller to the control computer. The control computer will use thisinformation to disable sensors that are not over the part surface. Thecontrol computer also receives a listing of active tows being placedfrom the robot controller and is programmed with the number of plies inthe current layup. In accordance with one embodiment, the controlcomputer is configured to execute a PID loop to control the temperatureat the compaction point (e.g., at the interface of the compaction rollerand a newly laid tow) and regulate the heater power to achieve thedesired compaction point temperature.

Although various embodiments of systems and methods for controlling thetemperature at the compaction point in an AFP machine will be describedin some detail below, one or more of those embodiments may becharacterized by one or more of the following aspects.

One aspect of the subject matter disclosed in detail below is anautomated fiber placement machine comprising: a head comprising acompaction roller; a heater mounted forward of the compaction roller; afirst temperature sensor directed at a first measurement spot locatedeither forward of the compaction roller and aft of the heater or aft ofthe compaction roller, wherein the first temperature sensor, whenoperative, outputs first temperature data representing an amount ofradiation transduced into electrical signals by the first temperaturesensor when the compaction roller is in contact with a substrate; anon-transitory tangible computer-readable storage medium storingcomputer code representing a thermal model that is configured to inferan estimated compaction point temperature of the substrate under thecompaction roller based at least in part on temperature data output byone or more temperature sensors; and a computing system configured toperform the following operations: using the thermal model to calculatean amount of electrical power to be supplied to the heater as a functionof at least the first temperature data output by the first temperaturesensor; and outputting heater power control signals representing theamount of electrical power to be supplied to the heater. The automatedfiber placement machine may further comprise a second temperature sensordirected at a second measurement spot located either forward of thecompaction roller and aft of the heater if the first measurement spot islocated aft of the compaction roller or aft of the compaction roller ifthe first measurement spot is located forward of the compaction rollerand aft of the heater, wherein the second temperature sensor, whenoperative, outputs second temperature data representing an amount ofradiation transduced into electrical signals by the second temperaturesensor when the compaction roller is in contact with the substrate,wherein the thermal model is configured to infer the estimatedcompaction point temperature of the substrate under the compactionroller based at least in part on the first and second temperature data.In accordance with one proposed embodiment, the computing system isconfigured to calculate the amount of electrical power to be supplied tothe heater as a function of a difference between the first and secondtemperature data output by the first and second temperature sensors.

In accordance with some embodiments of the system described in thepreceding paragraph, the heater comprises an infrared heater, while thefirst and second temperature sensors comprise first and second infraredtemperature sensors respectively. The head may further compriseshielding disposed and configured to block radiation reflected by thesubstrate from reaching the first temperature sensor.

In accordance with one embodiment, the thermal model is configured totake into account a speed at which the head is moving and a number ofplies of the substrate. Furthermore, the thermal model may be configuredto calculate a difference between the estimated compaction pointtemperature and a target compaction point temperature.

Another aspect of the subject matter disclosed herein is a method forcontrolling a heater during placement of tows of fiber-reinforcedplastic material by a fiber placement machine. The method comprises: (a)creating a thermal model that correlates a temperature of a compactionpoint under a compaction roller to at least a first temperature of asubstrate in a first measurement spot, wherein the first measurementspot is located either forward of the compaction roller and aft of theheater or aft of the compaction roller when the compaction roller is incontact with the substrate; (b) compacting tows of fiber-reinforcedplastic material on the substrate by rolling the compaction roller on asurface of the substrate with the tows therebetween; (c) heating thesubstrate in an area upstream of the first measurement spot duringcompaction using an electrically powered heater; (d) acquiring a firsttemperature measurement from the first measurement spot; (e) using thethermal model to infer an estimated compaction point temperature that isa function of at least the first temperature measurement; (f)calculating a difference between the estimated compaction pointtemperature and a target compaction point temperature; (g) issuingcontrol signals that represent a command to supply an amount ofelectrical power to the heater, which amount of electrical power iscalculated to reduce the difference between the estimated compactionpoint temperature and the target compaction point temperature; and (h)supplying the amount of electrical power to the heater, wherein steps(e) through (g) are performed by a computing system.

In accordance with some embodiments of the method described in thepreceding paragraph, the thermal model also correlates the temperatureof the compaction point to a second temperature of the substrate in asecond measurement spot, wherein the second measurement spot is locatedforward of the compaction roller and aft of the heater if the firstmeasurement spot is aft of the compaction roller or aft of thecompaction roller if the first measurement spot is forward of thecompaction roller and aft of the heater when the compaction roller is incontact with the substrate, and step (e) comprises using the thermalmodel to infer an estimated compaction point temperature that is afunction of a difference of the first and second temperaturemeasurements.

A further aspect is a method for controlling a heater during placementof tows of fiber-reinforced plastic material by a fiber placementmachine, comprising: (a) compacting tows of fiber-reinforced plasticmaterial on a substrate supported by a tool by rolling a compactionroller on a surface of the substrate with the tows therebetween; (b)heating the substrate in an area upstream of the compaction roller usingan electrically powered heater; (c) acquiring a first temperaturemeasurement from a first measurement spot on a portion of the substratelocated aft of the heater and forward of the compaction roller; (d)acquiring a second temperature measurement from a second measurementspot on a portion of the substrate located aft of the compaction roller;(e) inferring an estimated compaction point temperature that is afunction of at least one of the first and second temperaturemeasurements; (f) calculating a difference between the estimatedcompaction point temperature and a target compaction point temperature;(g) issuing control signals that represent a command to supply an amountof electrical power to the heater, which amount of electrical power iscalculated to reduce the difference between the estimated compactionpoint temperature and the target compaction point temperature; and (h)supplying the amount of electrical power to the heater, wherein at leaststeps (e) through (g) are performed by a computing system. In accordancewith some embodiments, step (c) comprises radiating the substrate withinfrared radiation. In accordance with one example implementation, themethod further comprises calculating the amount of electrical power tobe supplied to the heater as a function of a difference between thefirst and second temperature data output by the first and secondtemperature sensors.

Other aspects of systems and methods for controlling the temperature atthe compaction point in an AFP machine are disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, functions and advantages discussed in the precedingsection can be achieved independently in various embodiments or may becombined in yet other embodiments. Various embodiments will behereinafter described with reference to drawings for the purpose ofillustrating the above-described and other aspects. None of the diagramsbriefly described in this section are drawn to scale.

FIG. 1 is a diagram showing a side view of a head of an AFP machine inthe process of laying a tow of fiber-reinforced plastic material on asubstrate. The head comprises a compaction roller, an infrared heaterand IR temperature sensors.

FIG. 2 is a block diagram identifying some components of a closed-loopsystem for controlling the temperature at the compaction point in an AFPmachine in accordance with one embodiment.

FIG. 3 is a diagram representing a sectional view of a polymeric (e.g.,polyurethane) compaction roller being deformed in the compaction zone asit presses against a composite substrate laid on a tool made of metal(e.g., aluminum). Three points of interest are indicated as follows:A—aft roller sensing location; B—forward roller sensing location; andC—compaction point under roller.

FIG. 4 is a flowchart identifying steps of an infrared heater mastercontrol process in accordance with one embodiment.

FIG. 5 is a block diagram identifying some data inputs and somecomponents of a control computer configured to control an infraredheater in accordance with one embodiment.

FIG. 6 is a diagram showing the same side view of the AFP headpreviously depicted in FIG. 1, except that in the scenario depicted inFIG. 6, the tool is further away from the infrared heater.

FIG. 7 is a diagram showing the same side view of the AFP headpreviously depicted in FIG. 1, except that in the scenario depicted inFIG. 7, the tool is closer to the infrared heater.

FIG. 8 is a graph showing a conceptualized temperature history at anypoint on the layup course.

FIG. 9 is a graph of substrate temperature versus time during heating,cooling and roller compaction based on buried thermocouple data.

FIG. 10 is a graph showing a conceptualized time history of heat flux oneach surface element.

FIG. 11 is a graph showing temperature-time histories for the threepoints of interest (A, B and C) indicated in FIG. 3

FIG. 12 is a graph showing temperature-time histories at the compactionpoint for heater (i.e., head) speeds of 0.05 m/sec, 0.1 m/sec and 0.25m/sec.

FIG. 13 is a graph showing temperature-time histories at the compactionpoint for heater power output percentages of 20%, 40%, 60% and 80%(heater speed of 0.1 m/sec).

FIG. 14 is a graph showing the ratio of the temperature differencebetween points C and A shown in FIG. 3 over the temperature differencebetween points B and A shown in FIG. 3 as a function of number of plieslaid down (heater speed of 0.1 m/sec).

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

For the purpose of illustration, systems and methods for closed-loopcontrol of the temperature at the compaction point in an AFP machinewill now be described in some detail. However, not all features of anactual implementation are described in this specification. A personskilled in the art will appreciate that in the development of any suchembodiment, numerous implementation-specific decisions must be made toachieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

The particular exemplary embodiments disclosed below are based on thefollowing overall control philosophies: (1) IR temperature sensors areused to detect substrate temperature; (2) mounting and shielding arecontrolled to prevent reflected photons from impinging on the IRtemperature sensors; (3) the substrate temperature is characterized as afunction of heater power, compaction roller velocity and number of pliesin the substrate; (4) a thermal model is developed which relatesmeasured temperatures to actual process temperatures; (5) an open loopcontrol scheme is run when the temperature data is not valid; (6) aclosed loop control scheme is run to trim temperature when thetemperature data is valid; and (7) live data from the robot andnumerical control (NC) programming are also used as process controls.

In accordance with one embodiment, the inputs to the controller systeminclude the following: (1) at least one pyrometer (hereinafter“temperature sensor”) to measure material temperature near thecompaction roller (more than one can be used to measure in front of theroller, behind the roller, and at the left and right edges of theroller); (2) a thermal model that predicts the material temperaturebased on heater output, process speed, tooling material, number of pliesover the tool, distance of the heater to the substrate, and orientationof the heater to the substrate; (3) NC program data that passes theorientation of the heater to the substrate to account for complexcontours; (4) robot status data for speed, position, and acceleration;(5) number of plies previously placed, as presented by the NC program;(6) substrate material emissivity; (7) NC program data defining whichtows are being processed by the head; (8) compaction roller temperature;and (9) substrate temperatures as measured by the temperature sensors.The control computer processes these inputs and outputs control signalsrepresenting the commanded heater power level.

FIG. 1 is a diagram showing a side view of a head 10 of a robotic AFPmachine in the process of laying a tow 14 of fiber-reinforced plasticmaterial on a substrate 16 in accordance with one embodiment. The headis moving in the direction indicated by a horizontal arrow in FIG. 1.The head 10 comprises a compaction roller 12 and is equipped with aninfrared heater 20 (e.g., a plurality of infrared bulbs) and amultiplicity of IR temperature sensors mounted in strategic locations.The multiplicity of IR temperature sensors includes a first row ofspaced-apart IR temperature sensors 22 (only one of which is visible inFIG. 1) in front of the compaction roller 12 and a second row ofspaced-apart IR temperature sensors 24 (only one of which is visible inFIG. 1) behind the compaction roller 12. In accordance with oneembodiment, both rows have three IR temperature sensors. The IRtemperature sensors 22 measure the substrate temperature after heatingand before compaction; the IR temperature sensors 24 measure thesubstrate temperature after heating and after compaction. Other IRtemperature sensors for measuring the substrate temperature beforeheating and any changes in the temperature of the compaction roller arenot shown in FIG. 1.

A typical IR temperature sensor comprises a lens, a spectral filter thatselects the wavelength spectrum of interest, an optical detector thatconverts the infrared radiation into an electrical signal, and anelectronic signal processing unit that analyzes the electrical signaland converts it into a temperature measurement. Such an IR temperaturesensor has the capability to measure temperature without touching anobject. For example, suitable IR temperature sensors are commerciallyavailable from Fluke Process Instruments N.A., Santa Cruz, Calif.

Reflection of infrared energy from the substrate 16 into the IRtemperature sensors 22 would prevent adequate control. To avoid thissituation, bulb shielding 26 is disposed and configured to blockinfrared energy reflected by the substrate 16 from reaching the IRtemperature sensors 22. More specifically, the infrared heater 20 isattached to bulb shielding 26, which is in turn attached to the head 10of the AFP machine. Preferably the optical detector of each of the IRtemperature sensors 22 is capable of forming a reasonable measurementspot size given the close mounting position and the desire to minimizeobservation of reflected energy.

The substrate 16 comprises a multiplicity of tows of fiber-reinforcedplastic material previously laid on a tool 18 (e.g., a mandrel). Thetows form plies of composite material. As the head 10 moves in thedirection indicated by the arrow in FIG. 1, one or more additional tows14 are laid on the substrate 16. The infrared bulbs 20 in front of thecompaction roller 12 heat the substrate 16 in order to enhance materialtack prior to laminating a new ply over the substrate 16. The plies ofcomposite material will be cured in an autoclave after the layup processhas been completed.

The primary variable that controls part quality for fiber placement isthe substrate temperature under the compaction roller 12 duringcompaction (hereinafter “compaction point temperature”). However, thecompaction point temperature cannot be measured in real-time. Themethodology for heating proposed herein infers (i.e., estimates) thesubstrate temperature under the compaction roller 12. More specifically,2-D thermal modeling is used to correlate the compaction pointtemperature to the temperature readings of the IR temperature sensors 22and 24. The proposed methodology controls this primary parameter (i.e.,compaction point temperature) in a closed-loop heater control systemsuch that optimal processing conditions can be maintained during thefabrication of composite structures in order to provide the optimalquality at the highest layup rates possible. More specifically, theclosed-loop heater control system disclosed herein controls the heateroutput based on a combination of IR temperature sensor feedback, processmodels, NC programming data, ambient conditions, and material models todetermine the optimal electrical power to supply to the infrared heater20.

In accordance with one implementation adopted during development, eightIR temperature sensors were mounted to the head 10 of a robotic AFPmachine. A first set of three IR temperature sensors 22 were mounted infront of the compaction roller 12. A second set of three IR temperaturesensors 24 were mounted behind the compaction roller 12. Each IRtemperature sensor of the first and second sets was pointed at arespective measurement spot on the substrate being processed. Thesemeasurement spots were located as close to the compaction roller 12 aspossible. A seventh IR temperature sensor (not shown in FIG. 1) wasmounted out in front of the infrared heater 20 to measure the substratetemperature before heating. The last IR temperature sensor (not shown inFIG. 1) was mounted near the compaction roller 12 and positioned so asto measure any changes in the temperature of the compaction roller 12.

The IR temperature sensors 22 between the compaction roller 12 and theinfrared heater 20 enable close monitoring of substrate temperatures,which can increase as they accrue repeated heating cycles. The IRtemperature sensors 24 behind the compaction roller 12 provide areliable, reflection-free reading that is a lagging indicator of heaterresponse. The IR temperature sensors 22 and 24 are preferably located sothat they are between ±45 degrees of perpendicular to the surface. Inaddition, the compaction point is the region of interest, so pointingthe sensors as close as possible to the nip improves the systemperformance.

In accordance with various embodiments of the heater control systemdisclosed herein, the number of temperature sensors may be differentthan eight. For example, the thermal model may be configured to infer anestimated compaction point temperature as a function of one or moretemperature sensors directed at respective measurement spots locatedforward of the compaction roller and aft of the heater (and not as afunction of any temperature sensors directed at respective measurementspots located aft of the compaction roller). In an alternative example,the thermal model may be configured to infer an estimated compactionpoint temperature as a function of one or more temperature sensorsdirected at respective measurement spots located aft of the compactionroller (and not as a function of any temperature sensors directed atrespective measurement spots located forward of the compaction rollerand aft of the heater).

FIG. 2 is a block diagram identifying some components of a closed-loopsystem for controlling the temperature at the compaction point in an AFPmachine. The overall system comprises a control computer 2 forcontrolling the infrared heater 20 and a robot controller 4 forcontrolling movement of the head 10 of the AFP machine. The robotcontroller 4 provides data to the control computer 2 over a networkconnection (e.g., an Ethernet connection). The robot controller 4 isprogrammed to provide speed and tow control output codes over thenetwork connection to the control computer 2. In accordance with oneembodiment, the control computer 2 is wired to a multiplexer 28 to readthe IR temperature sensors 22, 24. The IR temperature sensors are alsowired into the multiplexer 28.

The control computer 2 reads temperature data, robot data, and partprogram information and outputs heater power control signals thatcontrol the power supplied to the infrared heater 20 in a closed-loopcontrol system. The heater power control signals are sent by the controlcomputer 2 to a signal conditioner 6, which in turn outputs conditionedheater power control signals to a heater power controller 8. The heaterpower controller 8 is configured to convert conditioned heater powercontrol signals to an output voltage which is used to power the infraredheater 20.

The control computer 2 is configured to employ a thermal model, in formof a set of algebraic equations, which incorporates respective thermalanalysis results of the geometry and thermal properties of the tooling,layup, compaction roller and heater and their thermal interactions. Thethermal model receives input variables (including the IR temperaturesensor readings) and then estimates the substrate temperature at theinterface of the compaction roller 12 and the substrate 16 (hereinafter“the compaction point” or “nip”). The input variables include tool/layupmodel data such as the number of plies under the current layup (toaccount for variations in thermal flow), head (e.g., compaction roller)geometry data, temperature data (from the IR temperature sensors 22, 24and optionally from the IR temperature sensor measuring the temperatureof the compaction roller 12), and layup speed (supplied by the robotcontroller 4 continuously throughout the layup). In addition, the partand head geometries influence which, if any, IR temperature sensors 22and 24 are pointed at the part. This data will be sent from the robotcontroller 4 to the control computer 2. The control computer 2 will usethis information to disable IR temperature sensors that are not over thepart surface and, therefore, not capturing meaningful data.

FIG. 3 is a diagram representing a sectional view of a polymeric (e.g.,polyurethane) compaction roller 12 being deformed in the compaction zoneas it presses against a composite substrate 16 laid on a tool 18 made ofmetal (e.g., aluminum). Three points of interest are indicated in FIG.3: A—aft roller sensing location; B—forward roller sensing location; andC—compaction point under the roller. The portions of the compactionroller 12, substrate 16 and tool 18 inside the dashed rectangle in theupper portion of FIG. 3 are shown on a magnified scale in the lowerportion of FIG. 3. In this magnified portion, it can be seen that theportion of the outer circumference of the compaction roller 12 thatcontacts the substrate 16 in the compaction zone will become flattened.The temperature of interest, which is derived using the thermal model,is the temperature at the compaction point.

The thermal model assumes that the thermal process has already reachedsteady state. Hence, every point on the tape-laying path experiences thesame temperature history, except for the absolute start time at eachpoint. However in reality, steady state is not always true. For example,during the initial start-up paths when everything is cold (at ambienttemperature). Also, other factors (such as substrate and rollertemperatures) usually do not remain constant during the whole processand are difficult to account for in a 2-D steady-state thermal model.Accordingly, a differential approach was adopted using an empiricallyderived ratio R (approximately constant over the temperature ranges ofinterest) which equals the difference between the substrate temperaturesat points A and C and the difference between the substrate temperaturesat points A and B (i.e., R=(T_(C)−T_(A))/(T_(B)−T_(A))) to minimizeerrors from the model due to initial condition changes from time to timeand path to path. In accordance with one proposed implementation, thethermal model uses the ratio R to estimate the compaction pointtemperature T_(C) in response to input of all of the previouslyidentified variables, including the current heater power and sensorreadings (i.e., measured temperatures) T_(A) and T_(B). The thermalmodel is further configured to calculate a target heater power based onthe deviation of the estimated compaction point temperature T_(C) from apre-specified compaction point temperature (selected from pre-storedcurves) determined to be the optimal value for the applicable processingconditions.

The thermal model was used to develop control algorithms relating thespeed, number of plies and heater power to the layup temperature underthe compaction roller 12. These control algorithms are programmed intothe control computer 2. In accordance with one embodiment, the controlalgorithms are implemented using a PID controller (incorporated in thecontrol computer 2). A PID controller is a control loop feedbackmechanism that continuously calculates an error value as the differencebetween a desired setpoint (e.g., a target compaction point temperature)and a measured process variable (e.g., the estimated compaction pointtemperature) and applies a correction based on proportional, integral,and derivative terms. In this case, the PID controller continuouslycalculates a difference between the current heater power and a targetheater power determined using the temperature difference from thethermal model. The gains for the PID controller are tuned to ensuresatisfactory control of the power supplied to the infrared heater 20.For example, those gains are tuned to address the thermal lag of thesensors and heater.

As can be seen in FIG. 1, the bulbs of the infrared heater 20 are spaceda certain distance apart. The irradiance pattern is roughly cylindricalfor each element in each double bulb, so depending on the distance fromthe infrared heater 20, the overall heated area will vary, resulting invariation in the overall or average power density as a function of area.Testing was conducted to determine the energy absorption of thesubstrate at different power settings, substrate thicknesses, anddelivery head feed rates. Using the mounted IR temperature sensors, aseries of constant-velocity, constant-power tests were run over avarying number of substrate plies. Temperature data was acquired fromrespective areas before and after the compaction roller 12, both whenlaying material and when not laying material. The data collected inthese tests was used to calibrate the efficiency of the infrared heater20 over the entire heated zone as a function of power input and feedrate. This data was also used to derive the parameters for slope andintercept to determine heat input as a function of feed rate and targettemperature.

While not directly related to the heater characterization, thecompaction roller 12 makes a significant contribution to the substratetemperature during processing. The actual processing temperature duringfiber placement (that is temperature at the compaction point) is notwhen the material is at its warmest point. Since the compaction roller12 is separated from the infrared heater 20 by a distance (see FIG. 1),there is a period of cooling that occurs before the compaction roller 12compresses the new material onto the substrate 16 at the compactionpoint. This is the point at which the AFP process actually takes place.A test was developed to characterize the location of the compactionprocess in the temperature versus time plots of a single point along acourse. This test involved placing a thermocouple (not shown in thedrawings) along the centerline of a course and locating a compactionroller-actuated switch upstream a known distance, which produces atiming signal to calculate the position of the compaction roller 12relative to the thermocouple data. A course was run over the test setupwith the infrared heater 20 running in constant-power mode. Substratetemperature and the timing switch data were collected for the singlecourse with the compaction roller 12 in contact with the substrate 16under normal compaction loads. Data was collected on a second, identicalpass, except that the compaction roller 12 was not in contact with andjust barely above the surface of the substrate 16, to minimize effectsof increased distance to the substrate. The results of this test aredepicted in FIG. 9 (discussed in more detail below).

As previously described, the control computer 2 is programmed to controlthe power supplied to the infrared heater 20 during the laying down oftows and compaction of the substrate. FIG. 4 is a flowchart identifyingsteps of an infrared heater master control process 100 in accordancewith one embodiment. The first branch point in this flow is the decisionpoint 102, where a determination is made whether a special function isrequired or not. This branch handles those situations when the basicsensor feedback loop cannot be used. For example, when the sensors arenot pointed directly at the layup, a special function Sensor Off PartMethod 116 is employed. When the IR temperature sensors cannot be used,the control computer for the heater control system reverts to thetraditional method using speed-based open-loop equations for powercontrol. When the layup is starting a course add routine, a specialfunction such as Course Start Method 112 or Course Restart Method 114 isemployed.

The left branch in the flowchart seen in FIG. 4 is the method used whenthe sensors are valid (i.e., no special function is required). In thisloop, the system acquires speed and tow status data from the robotcontroller and temperature data from the IR temperature sensors (step104). This information is channeled to the thermal model equations alongwith the number of plies in the current layup (step 106).

Initially, heaters may be in operation at a low power (for example,during startup). After receiving all relevant data, the thermal model(in form of algebra equations in the system) infers (i.e., estimates)the compaction point temperature based on the temperature readings fromthe forward and aft IR temperature sensors 22 and 24 (step 108) andoutputs a heater power level that should produce a desirable(pre-specified) compaction point temperature (hereinafter “targetcompaction point temperature”). Now the heater can operate at the powerlevel dictated by the thermal model. However, as temperature readingscontinue to be received during heating of the substrate and operation ofthe AFP machine, the thermal model will continuously record and outputsignals representing the current difference (i.e., “DELTA” in FIG. 4)between the thermal model-predicted (i.e., estimated) compaction pointtemperature and the target compaction point temperature (step 110). ThePID loop 118 incorporated in the control algorithms uses those changingdeviations to continuously fine-tune (i.e., apply a small correction to)the heater power level to achieve or maintain the target compactionpoint temperature. The estimated compaction point temperature T_(C) iscalculated using the equation T_(C)=T_(A)+R*(T_(B)−T_(A)).

As shown in FIG. 4, the special functions at the start and end of acourse and during off-part motion are controlled in the open-loopmethod. Only when the head is laying material and temperature data isvalid is the closed-loop control implemented.

FIG. 5 is a block diagram identifying the data inputs to and algorithmsexecuted by a control computer 2 configured to output infrared heaterpower control signals in accordance with one embodiment. Thisclosed-loop heater control system uses a thermal model 40 (previouslydescribed) that can be loaded into the memory (e.g., random accessmemory) of the control computer 2. The thermal model 40 takes intoaccount a tool/layup model 42 (which provides the number of plies in thecurrent layup) and a head geometry model 44, both of which can beretrieved from a non-transitory tangible computer-readable storagemedium (not shown). The tool/layup model 42 includes information fromthe NC programming 48. In addition, the thermal model 40 (when loadedinto the control computer 2) receives robot data inputs 46 (such asspeed and position of the head 10, distance separating the heater andthe substrate, and the number of active tows) from the robot controller4 (see FIG. 2) and temperature data from the IR temperature sensors 22,24. The thermal model 40 comprises equations configured to outputtemperature estimates 50 based on the variables input to the thermalmodel 40. These temperatures estimates 50 include the inferred substratetemperature under the compaction roller 12 and the inferred substratetemperature under the infrared heater 20.

The thermal model 40 receives inputs from the IR temperature sensors(including IR temperature sensors 22, 24 and the other IR temperaturesensors). More than one IR temperature sensor can be used to measurematerial temperature in front of the compaction roller, behind thecompaction roller, and at the left and right edges of the compactionroller. In addition to the temperature measurements from the IRtemperature sensors, the thermal model 40 is configured to take intoaccount the number of plies previously placed, as presented by the NCprogramming 48, and the substrate material emissivity, which informationis included in the tool/layup model 42. To account for complex contoursin the tool 18, the control computer 4 receives robot data 46representing the real-time position, speed and acceleration of the AFPhead 10, the real-time distance and orientation of the infrared heater20 relative to the substrate, the number of active tows being processedby the AFP head 10. Using the received information, the thermal model 40estimates the substrate temperature under the compaction roller 12.

The control computer 2 further executes control algorithms 60 which areconfigured to maintain the optimal heating conditions for the fiberplacement of thermoset prepreg materials in response to receipt of thetemperature estimates 50. The control computer 2 processes the inputs inreal-time and outputs control signals indicating the heater power neededto reach the target substrate temperature at the compaction point. Thecontrol algorithms 52 include a PID controller. Based on the differencebetween the estimated and target compaction point temperatures, the PIDcontroller generates heater power control signals for changing theelectrical power supplied to the infrared heater 20 in a manner thatreduces the difference between the estimated and target compaction pointtemperatures. In addition, the estimated temperature under the infraredheater 20 can be used to prevent the material from exceeding maximumallowable temperatures.

In particular, the control computer 2 is configured to control theinfrared heater 20 based on the proximity of the tool 18. FIG. 6 is adiagram showing the same side view of the AFP head 10 previouslydepicted in FIG. 1, except that in the scenario depicted in FIG. 6, thetool 18 is further away from the infrared heater 20 due to the contourof the tool 18. Conversely, in the scenario depicted in FIG. 7, the tool18 is closer to the infrared heater 20.

The distance separating the tool 18 and infrared heater 20 at theclosest point (i.e., the minimal distance) can be calculated by therobot controller 4 (see FIG. 2) using information from the tool/layupmodel 42, information from the head geometry model 44 and head positiondata extracted from the NC programming 48 (which head position data is acomponent of robot data 46). To compensate for changes in the distancebetween tool 18 and infrared heater 20, the power supplied to thevariable-power infrared heater 20 should be adjusted by control computer2 to be inversely proportional to that distance. In accordance withalternative embodiments, the distance between tool 18 and infraredheater 20 can be measured using distance sensors (e.g., an opticaldetector head comprising an interferometer and a photodetector).

In accordance with one embodiment, there are three scenarios in whichtows are being dispensed from the head. The IR temperature sensors 22,24 need to be situated so that there is temperature data available tothe control computer 2 for each scenario. The first scenario is when acourse progresses down a tapered surface, and the outer tows are cut tonarrow the band width accordingly. The other two scenarios are mirrorimages of each other, when tows are actively being placed either at oneedge of the tow band or the other. For example, the last course at theend of a ply when the fiber path is parallel to the ply boundary, theremay only be a few tows being placed. In this case, the outer tows areused, not the center tows, unless specifically selected by the NCprogrammer. Sensors are needed to measure substrate temperatures whenthe outer tows are being placed. In a proposed implementation, a 12-towhead would be used. Therefore, the three IR temperature sensors of eachset (i.e., the set of IR temperature sensors 22 and the set of IRtemperature sensors 24) would be located to sense the respectivetemperatures of tow #1, between tows #7 and #8, and tow #12. The seventhIR temperature sensor would be pointed at the back side of thecompaction roller 12 to measure its temperature. And the last IRtemperature sensor would be placed ahead of the infrared heater 20 tomeasure the substrate temperature prior to active heating.

During a fiber placement process, not all IR temperature sensors will bein a position to provide a valid reading. At the start and end ofcourses, as the head approaches or retracts from the surface, there canbe reflections that are viewed by an IR temperature sensor. Also, theoutboard IR temperature sensors (for example, the sensors positioned tomeasure with tow #1 or tow #12 in a 12-tow band) may not be viewing thearea being processed because of the number of tows being placed.Additionally, at the beginning or end of the courses, the IR temperaturesensors could be viewing the tooling 18, producing a non-relevantresponse. In accordance with one embodiment, means for determining whenthe IR temperature sensor is providing a valid signal are incorporated.The post-processing will be configured to read the NC program and insertcodes for defining which sensors are producing valid signals.

Because there are three IR temperature sensors 22 in front of thecompaction roller 12 and three IR temperature sensors 24 behind thecompaction roller 12, means for integrating the signals of the three aftand three forward IR temperature sensors into a usable input for thecontrol algorithms 52 are included in the software. For example, if allthree IR temperature sensors 22 in front of the compaction roller 12 areproducing different temperatures, a method for producing a common signalto the control computer 2 is desirable. In accordance with onetechnique, the center IR temperature sensors can be used to drive thecontrol algorithm.

The purpose of thermal modeling is to provide the AFP machine with anaccurately predicted temperature at the compaction point in relation tothe before- and after-roller IR temperature sensor measurements. Atwo-dimensional finite element model (FEM) was built parametrically andanalyses were carried out to predict the compaction point temperature asa function of processing speed and power output of the infrared heater20. The FEM simulation also accounted for heat loss due to rollercontact and the effect of number of plies already laid up.

By considering the physical process of fiber placement, a conceptualizedtemperature history at any point on the layup course can be derived.FIG. 8 is a graph showing a conceptualized temperature history at anypoint on the layup course. The reading from the IR temperature sensors22 before the compaction roller 12 (indicated by the leftmost shaded dotin FIG. 8) is slightly to the right of the peak due to the laggingdistance between the sensing location and the trailing edge of theinfrared heater 20. Roller contact occurs at the change of the slopepoint and lasts for a certain time depending on the contact length andspeed. After roller contact, natural cooling via convection continuesand the reading from the IR temperature sensors 24 after the compactionroller 12 (indicated by the rightmost shaded dot in FIG. 8) is acquired.In summary, the task for thermal modeling is to analytically simulatethis temperature history curve and develop equations for predicting thecompaction point temperature, defined as the temperature at the startpoint of the cooling under the roller zone, as indicated in FIG. 8.Further details regarding the technical approach in simulating theactual process will be provided below.

The heater control system disclosed herein relies on open-loopalgorithms to manage the off-part heater controls as well as thetransition regions when material is being placed but the IR temperaturesensors are not providing valid data due to their locations, such as atthe start and end of a course. The control architecture also relies onthe open-loop algorithms to provide a target power setting based on theAFP head velocity, and the PID closed-loop controller varies the actualpower setting based on the target, to achieve the commanded substratetemperature. Infrared camera data was collected to understand theprofile of the heat-affected zone of the heater system installed on therobotic AFP system. In accordance with one prototype, the heater bulbswere 6 inches wide, while the delivery head was designed for twelve towsof ½-inch width, for a band width of 6 inches. The infrared imagesacquired showed that this heater did not produce a uniform temperatureacross the entire 6-inch band width. There was about a 40° C. differencein temperature from the center of the heater area to the outer edgewhere the outer tows are placed. Since this was a static test, theactual temperatures recorded were not representative of the actualsubstrate temperature expected during fiber placement, but were insteadan indicator of the heater uniformity. To mitigate this variability,infrared bulbs that are wider than the band width of the material can beutilized. In accordance with one embodiment, the thermal model can beconfigured to use the center IR temperature sensors exclusively.

At the end of the course, material temperatures drop and then spike whenthe AFP head 10 comes to the end of the course and is lifted off. Thisaction produces a temperature transient. The closed-loop system may beconfigured to ignore the incoming data when it is no longer valid, suchas when retraction of the AFP head 10 from the surface produces atemperature transient.

As part of the process of calibrating the thermal model, tests wereconducted to measure the temperatures of the areas detected by theforward center and aft center IR temperature sensors. There was asignificant drop in substrate temperature between the forward and aftsensors. The actual process occurs at the point of compaction and is thedesired point of control for the closed-loop system. However, that pointcannot be measured. For this reason, a thermal model was developed tounderstand the relationship between the sensor readings before and afterthe roller and the temperature at the compaction point. From the thermalmodel, an inferred compaction point temperature can be calculated andused to drive the heater control system to produce a desired compactionpoint temperature. The thermal model may be configured to replicate theconditions of actual testing. The temperature data acquired during thetest was also used to calibrate the heater efficiency values in thethermal model.

The previously mentioned testing was conducted while not laying tow.Some of the testing was repeated while laying tow. For a substrate offour or eight plies, the measured substrate temperature varied by anumber of degrees Celsius compared to when not laying tow. This data wasalso used to calibrate the thermal model.

Another example of the effects of the compaction roller 12 is shown inFIG. 9, which is a graph of substrate temperature versus time duringheating, cooling and roller compaction based on buried thermocoupledata. The plots show the substrate temperature as measured by athermocouple on the ply below the surface, as the infrared heater 20 andcompaction roller 12 pass over the thermocouple. The line labeled “Withcompaction” is data collected with the roller in contact with thesurface. The line labeled “Without compaction” is data collected withthe roller just barely off the surface. The two vertical lines representthe boundaries of the time interval (bounded by a “Compaction start”time and a “Compaction stop” time) during which the material was beingcompacted by the roller. The arrows approximate where the forward andaft IR temperature sensors are pointed. In this plot, the temperature atthe compaction point quickly drops when the material is under compactionby the roller. Here the roller is quenching the material. When re-runwithout roller contact, no quenching occurred. Also evident in this plotis that the substrate temperature begins to cool prior to compaction,due to the proximity of the heater to the compaction point.

Given the two infrared-bulb configuration of the infrared heaterdepicted in FIG. 1, it is reasonable to assume that the heat flux at thecenter of the heated compaction zone is higher than that at the edges.In constructing a thermal model for the purpose of initial development,the approximated spatial heat flux distribution was further approximatedinto a heat flux time history that every surface element experienceswhen the heater moves from left to right. FIG. 10 is a graph showing aconceptualized time history of heat flux on each surface element. Inthis developmental example, the average heat flux was H, H1=0.5H,H2=1.5H, and H2=3H1. The parameter t₁ is the delay time that controlsthe beginning of heating according to the location of a particularsurface element and the heater moving speed. The parameter t₄ is thedelay time that controls the end of heating. The heat flux peaking timet_(p)=t₁+L_(H)/(2 V), where L_(H) is the heating length and V is thespeed of the moving heater. Δt=t₂−t₁=t₄−t₃ is a negligible ramp time forcomputational purposes only. An empirical approach was taken to derivethe relationship between the heater power output and the heat flux,which was found to be roughly linear. Through running the actual processat different speeds, an empirical efficiency coefficient C(V, d) wasobtained that regulates the actual heat flux applied to the thermalmodel, where d is the distance between the lamp surface and the surfaceof the top ply of the substrate. In accordance with one proposedembodiment of a method of characterizing the heat flux profile, athermal profile was designed which is focused on calculating the heatprofile based on bulb characteristics like view factor, number of bulbs,curvature of tool, and distance of heater to tool.

FEM analyses were carried out to predict the compaction pointtemperature as a function of processing speed and power output of theinfrared heater 20. The FEM simulation also accounted for heat loss dueto roller contact and the effect of the number of plies already laid up.FIG. 11 shows temperature-time histories at the three points of interest(A, B and C, see FIG. 3) when the heater was moving at 0.1 m/sec andpower output was 100%. The forward roller sensing location B curve isbasically an offset from the compaction point C curve before the rollercontact, which is only introduced at compaction point C. The aft rollersensing location A is cooler than compaction point C and forward rollersensing location B because it is on top of the prepreg ply just laiddown with the same initial temperature as the ambient temperature.

FIG. 12 shows the effect of processing speed on the compaction pointtemperature (point of interest C), with 100% power output. As expected,when processing speed increases from 0.05 m/sec to 0.1 m/sec and 0.25m/sec, the compaction point temperature decreases. (These low speedswere selected for the purpose of simulation only; fiber placementtypically occurs at faster speeds, e.g., 0.5 or more meters per second.)FIG. 13 demonstrates the effect of power output on compaction pointtemperature, which scales linearly with the power output percentage.Again, the data shown in FIGS. 12 and 13 were derived using FEMsimulation.

The parametric study results for a compaction point temperature controlsystem in accordance with one FEM simulation led to the followingconclusions: (a) temperature rise at each of the points of interest islinearly proportional to power output; (b) compaction point (C)temperature is closer to the aft roller sensing point (B) due to rollercontact cooling and the fact that point B is located closer tocompaction point C than point A; (c) there is essentially no differencewhen the number of laid-up plies is more than four because thethrough-the-thickness transient heat conduction within the time durationof interest does not go beyond eight plies for the given CFRP material;and (d) as a result, the ratio of the temperature difference betweenpoints C and A over the temperature difference between B and A as afunction of number of plies laid down approaches a constant (see FIG.14).

To validate the thermal model, heater characterization test results wereused to correlate with model predictions for the one-ply prepreg heatingtest. After further investigation, it was concluded that the finiteelement analysis results were consistent with theory—temperature riseshould be proportional to heat flux input.

The modeled compaction point temperature data was developed into amethod of predicting the percent power needed to the infrared heater asa function of number of plies in the substrate, feed rates, and targettemperatures. The method that was implemented assumed a fixed ratio ofthe difference between the compaction point temperature and the aft IRtemperature sensor temperature divided by the difference between theforward IR temperature sensor and aft IR temperature sensor. Using thisfixed ratio, the compaction point temperature can then be inferredknowing the difference in temperature between the forward and aft IRtemperature sensors.

The closed-loop feedback system described above is intended to controlfor extraneous variables. The substrate temperature is one suchvariable. Changes in the substrate temperature will be reflected in theIR temperature sensor readings and will then bring about changes in theheater power to achieve the desired compaction point temperature.

In summary, the system described above enables practice of a method forcontrolling a heater 20 during placement of tows of fiber-reinforcedplastic material by a fiber placement machine. In accordance with oneembodiment, the method comprises: (a) creating a thermal model 40 thatcorrelates a temperature of a compaction point under a compaction roller12 to first and second temperatures of a substrate 16 in first andsecond measurement spots respectively, wherein the first measurementspot is located forward of the compaction roller 12 and aft of theheater 22 and the second measurement spot is located aft of thecompaction roller 12 when the compaction roller 12 is in contact withthe substrate 16; (b) compacting tows of fiber-reinforced plasticmaterial on the substrate 16 by rolling the compaction roller 12 on asurface of the substrate 16 with the tows therebetween; (c) heating thesubstrate 16 in an area upstream of the first measurement spot duringcompaction using an electrically powered heater 20; (d) acquiring afirst temperature measurement from the first measurement spot; (e)acquiring a second temperature measurement from the second measurementspot; (f) using the thermal model 40 to infer an estimated compactionpoint temperature that is a function of at least one of the first andsecond temperature measurements; (g) calculating a difference betweenthe estimated compaction point temperature and a target compaction pointtemperature; (h) issuing control signals that represent a command tosupply an amount of electrical power to the heater 20, which amount ofelectrical power is calculated to reduce the difference between theestimated compaction point temperature and the target compaction pointtemperature; and (i) supplying the amount of electrical power to theheater, wherein steps (f) through (g) are performed by a computingsystem (e.g., control computer 4). This method may further comprise:acquiring a third temperature measurement from a third measurement spoton the substrate 16 located forward of the heater 20; configuring thethermal model 40 to infer an estimated heating point temperature of theportion of the substrate 16 under the heater 20 based at least in parton the difference between the first and third temperatures; determiningwhether the estimated heating point temperature exceeds a maximumallowable substrate temperature or not; and turning off the heater 22 ifthe estimated heating point temperature exceeds the maximum allowablesubstrate temperature.

While methods for closed-loop control of AFP heating have been describedwith reference to various embodiments, it will be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted for elements thereof without departing from the scope ofthe teachings herein. In addition, many modifications may be made toadapt the teachings herein to a particular situation without departingfrom the scope thereof. Therefore it is intended that the claims not belimited to the particular embodiments disclosed herein.

The embodiments disclosed above use one or more computing systems. Asused in the claims, the term “computing system” comprises one or moreprocessing or computing devices. Such processing or computing devicestypically include one or more of the following: a processor, acontroller, a central processing unit, a microcontroller, a reducedinstruction set computer processor, an application-specific integratedcircuit, a programmable logic circuit, a field-programmable gated array,a digital signal processor, and/or any other circuit or processingdevice capable of executing the functions described herein. The aboveexamples are exemplary only, and thus are not intended to limit in anyway the definition and/or meaning of the term “computing system”.

The methods described herein may be encoded as executable instructionsembodied in a non-transitory tangible computer-readable storage medium,including, without limitation, a storage device and/or a memory device.Such instructions, when executed by a processing or computing system,cause the system device to perform at least a portion of the methodsdescribed herein.

The process claims set forth hereinafter should not be construed torequire that the steps recited therein be performed in alphabeticalorder (any alphabetical ordering in the claims is used solely for thepurpose of referencing previously recited steps) or in the order inwhich they are recited unless the claim language explicitly specifies orstates conditions indicating a particular order in which some or all ofthose steps are performed. Nor should the process claims be construed toexclude any portions of two or more steps being performed concurrentlyor alternatingly unless the claim language explicitly states a conditionthat precludes such an interpretation.

The invention claimed is:
 1. An automated fiber placement machinecomprising: a head comprising a compaction roller; a tool disposed toform a compaction zone between the tool and the compaction roller; aheater mounted forward of the compaction roller for heating a substrateon the tool; a first temperature sensor directed at a measurement spoton the substrate located either forward of the compaction roller and aftof the heater or aft of the compaction roller, wherein the firsttemperature sensor, when operative, outputs temperature data when thecompaction roller is in contact with the substrate; a non-transitorytangible computer-readable storage medium storing computer coderepresenting a thermal model that is configured to estimate an estimatedcompaction point temperature of the substrate under the compactionroller based at least in part on the temperature data output by thetemperature sensor; and a computing system configured to performoperations comprising: using the thermal model to calculate an amount ofelectrical power to be supplied to the heater as a function of at leastthe temperature data output by the temperature sensor; and outputtingheater power control signals representing the amount of electrical powerto be supplied to the heater, wherein the computing system is furtherconfigured to execute an open-loop control algorithm when the head islaying material and the temperature data is invalid and a closed-loopcontrol algorithm when the head is laying material and the temperaturedata is valid.
 2. The automated fiber placement machine as recited inclaim 1, further comprising a second temperature sensor directed at asecond measurement spot on the substrate located either forward of thecompaction roller and aft of the heater if the first measurement spot islocated aft of the compaction roller or aft of the compaction roller ifthe first measurement spot is located forward of the compaction rollerand aft of the heater, wherein the second temperature sensor, whenoperative, outputs second temperature data when the compaction roller isin contact with the substrate, wherein the thermal model is configuredto estimate the estimated compaction point temperature of the substrateunder the compaction roller based at least in part on the first andsecond temperature data.
 3. The automated fiber placement machine asrecited in claim 2, wherein the heater comprises an infrared heater,while the first and second temperature sensors comprise first and secondinfrared temperature sensors respectively.
 4. The automated fiberplacement machine as recited in claim 2, wherein the computing system isconfigured to calculate the amount of electrical power to be supplied tothe heater as a function of a difference between the first and secondtemperature data output by the first and second temperature sensors. 5.The automated fiber placement machine as recited in claim 2, furthercomprising a third temperature sensor directed at a third measurementspot located forward of the heater, wherein the third temperaturesensor, when operative, outputs third temperature data when thecompaction roller is in contact with the substrate, and wherein thethermal model is further configured to estimate an estimated temperatureof the substrate under the heater based at least in part on a differencebetween the first and third temperature data output by the first andthird temperature sensors.
 6. The automated fiber placement machine asrecited in claim 1, wherein the head further comprises shieldingdisposed and configured to block radiation reflected by the substratefrom reaching the first temperature sensor.
 7. The automated fiberplacement machine as recited in claim 1, wherein the thermal model isconfigured to take into account a number of plies of the substrate. 8.The automated fiber placement machine as recited in claim 1, wherein thethermal model is configured to calculate a difference between theestimated compaction point temperature and a target compaction pointtemperature.
 9. The automated fiber placement machine as recited inclaim 8, wherein the computing system further comprises aproportional-integral-derivative controller that receives a signalrepresenting the difference calculated using the thermal model andoutputs heater power control signals configured to cause the heater tooperate in a manner that reduces the difference between the estimatedcompaction point temperature and the target compaction pointtemperature.
 10. The automated fiber placement machine as recited inclaim 1, further comprising: a signal conditioner operatively coupled toreceive the heater power control signals from the computing system; anda heater power controller operatively coupled to the signal conditioner,wherein the heater power controller is configured to convert conditionedheater power control signals to an output voltage which is used to powerthe heater.
 11. A method for controlling a heater during placement oftows of fiber-reinforced plastic material on a tool by a head of a fiberplacement machine, comprising: (a) creating a thermal model thatcorrelates an estimated temperature of a compaction point between a tooland a compaction roller to at least a first temperature of a substrateat a first measurement spot, wherein the first measurement spot is onthe substrate located either forward of the compaction roller and aft ofa heater or aft of the compaction roller when the compaction roller isin contact with the substrate; (b) compacting tows of fiber-reinforcedplastic material on the substrate by rolling the compaction roller on asurface of the substrate with the tows therebetween; (c) heating thesubstrate in an area upstream of the first measurement spot duringcompaction using the heater; (d) acquiring a first temperaturemeasurement from the first measurement spot; (e) using the thermal modelto estimate an estimated compaction point temperature that is a functionof at least the first temperature measurement; (f) calculating adifference between the estimated compaction point temperature and atarget compaction point temperature; (g) issuing control signals thatrepresent a command to supply an amount of electrical power to theheater, which amount of electrical power is calculated to reduce thedifference between the estimated compaction point temperature and thetarget compaction point temperature; (h) supplying the amount ofelectrical power to the heater; (i) acquiring a second temperaturemeasurement from a second measurement spot on the substrate, wherein thethermal model also correlates the temperature of the compaction point toa second temperature of the substrate at the second measurement spot,the second measurement spot is located forward of the compaction rollerand aft of the heater if the first measurement spot is aft of thecompaction roller or aft of the compaction roller if the firstmeasurement spot is forward of the compaction roller and aft of theheater when the compaction roller is in contact with the substrate, andstep (e) comprises using the thermal model to estimate an estimatedcompaction point temperature that is a function of a difference of thefirst and second temperature measurements; (j) determining whether thefirst and second temperature measurements are valid or not; and (k)executing an open-loop control algorithm when the first and secondtemperature measurements are not valid and a closed-loop controlalgorithm when the first and second temperature measurements are valid,wherein at least steps (e) through (g) are performed by a computingsystem.
 12. The method as recited in claim 11, further comprisingacquiring a second temperature measurement from a second measurementspot on the substrate, wherein the thermal model also correlates thetemperature of the compaction point to a second temperature of thesubstrate at the second measurement spot, the second measurement spot islocated forward of the compaction roller and aft of the heater if thefirst measurement spot is aft of the compaction roller or aft of thecompaction roller if the first measurement spot is forward of thecompaction roller and aft of the heater when the compaction roller is incontact with the substrate, and step (e) comprises using the thermalmodel to estimate an estimated compaction point temperature that is afunction of a difference of the first and second temperaturemeasurements.
 13. The method as recited in claim 11, wherein step (c)comprises radiating the substrate with infrared radiation.
 14. Themethod as recited in claim 12, further comprising blocking heatreflected by the substrate from reaching a temperature sensor that isdirected toward the one of the first and second measurement spots whichis located forward of the compaction roller and aft of the heater. 15.The method as recited in claim 11, wherein the estimated compactionpoint temperature outputted by the thermal model is a function of anumber of plies of the substrate.
 16. The method as recited in claim 11,further comprising: acquiring a third temperature measurement from athird measurement spot on the substrate located forward of the heater;configuring the thermal model to estimate an estimated heating pointtemperature of a portion of the substrate under the heater based atleast in part on the difference between the first and third temperaturemeasurements; determining whether the estimated heating pointtemperature exceeds a maximum allowable substrate temperature or not;and turning off the heater if the estimated heating point temperatureexceeds the maximum allowable substrate temperature.